Polycarbonate resin compositions and blends with other thermoplastic resins are widely used because of their excellent properties. Polycarbonate resin compositions and mixtures thereof with a saturated polyester and/or a polyester elastomer are described, for example, in European patent application No. 249,964. It is disclosed therein that very desirable improvements in chemical resistance, weather resistance and low temperature impact resistance are achieved if an additive comprising a silicone-based graft copolymer is used. In particular, there is used a modifier comprising a silicone rubber onto which a styrene/acrylonitrile co-monomer is grafted in the presence of a graft-linking agent. Such modifiers have relatively poor rubber integrity and incompatibility with the resins, and this may lead to poor surface appearance in the molded articles. Also relevant for its broad teachings is BASF's U.K. Patent No. 1,590,549 which also describes a silicone rubber graft polymer impact modifier for thermoplastics; poor compatibility is also observed with these compositions.
A novel polyorganosiloxane/polyvinyl-based graft polymer has now been discovered to be extremely useful as a modifier for such thermoplastic resins, and it does not show any tendency to cause delamination or surface mottling.
In the present invention, the polyorganosiloxane rubber substrate of the prior art is replaced by a co-homopolymerized network(s) of polyorganosiloxane/polyvinyl-based polymer(s) in a co-homopolymerization process. Polyorganosiloxane/polystyrene-based co-homopolymers are particularly preferred for use in the first stage of the graft polymers used in the present invention. The present invention uses graft polymers provided by subsequent graft polymerization of vinyl monomers (e.g., poly(methyl methacrylate), polystyrene or styrene/acrylonitrile copolymer) onto the polyorganosiloxane/polyvinyl co-homopolymer substrate as described.
This multi-stage graft polymer is usefully employed in the present invention as a superior impact strength modifier for thermoplastic molding compositions comprising polycarbonates and mixtures thereof with polyesters and/or poly(etherester) elastomers, poly(etherimide ester) elastomers or mixtures thereof; mixtures of polycarbonates, polyphenylene ethers, and polyesters; mixtures of polyphenylene ethers and polyesters; or mixtures of any of the foregoing. They maintain many unique features of silicone rubber, such as enhanced impact strength particularly at low temperature, and weld line strength, while maintaining other properties such as thermal resistance, but show no indication of delamination or mottled surfaces.